Pyrolytic decomposition of silicon-bearing gas in fluidized beds is an attractive process for producing polysilicon for the photovoltaic and semiconductor industries due to excellent mass and heat transfer, increased surface for deposition, and continuous production. Compared with a Siemens-type reactor, the fluidized bed reactor offers considerably higher production rates at a fraction of the energy consumption. The fluidized bed reactor can be highly automated to significantly decrease labor costs.
The manufacture of particulate polycrystalline silicon by a chemical vapor deposition method involving pyrolysis of a silicon-containing substance such as for example silane, disilane or halosilanes such as trichlorosilane or tetrachlorosilane in a fluidized bed reactor is well known to a person skilled in the art and exemplified by many publications including the following patents and publications: U.S. Pat. No. 8,075,692, U.S. Pat. No. 7,029,632, U.S. Pat. No. 5,810,934, U.S. Pat. No. 5,798,137, U.S. Pat. No. 5,139,762, U.S. Pat. No. 5,077,028, U.S. Pat. No. 4,883,687, U.S. Pat. No. 4,868,013, U.S. Pat. No. 4,820,587, U.S. Pat. No. 4,416,913, U.S. Pat. No. 4,314,525, U.S. Pat. No. 3,012,862, U.S. Pat. No. 3,012,861, US2010/0215562, US2010/0068116, US2010/0047136, US2010/0044342, US2009/0324479, US2008/0299291, US2009/0004090, US2008/0241046, US2008/0056979, US2008/0220166, US 2008/0159942, US2002/0102850, US2002/0086530, and US2002/0081250.
Silicon is deposited on particles in a reactor by decomposition of a silicon-bearing gas selected from the group consisting of silane (SiH4), disilane (Si2H6), higher order silanes (SinH2n+2), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), silicon tetrachloride (SiCl4), dibromosilane (SiH2Br2), tribromosilane (SiHBr3), silicon tetrabromide (SiBr4), diiodosilane (SiH2I2), triiodosilane (SiHI3), silicon tetraiodide (SiI4), and mixtures thereof. The silicon-bearing gas may be mixed with one or more halogen-containing gases, defined as any of the group consisting of chlorine (Cl2), hydrogen chloride (HCl), bromine (Br2), hydrogen bromide (HBr), iodine (I2), hydrogen iodide (HI), and mixtures thereof. The silicon-bearing gas may also be mixed with one or more other gases, such as hydrogen (H2) and/or one or more inert gases selected from nitrogen (N2), helium (He), argon (Ar), and neon (Ne). In particular embodiments, the silicon-bearing gas is silane, and the silane is mixed with hydrogen. The silicon-bearing gas, along with any accompanying hydrogen, halogen-containing gases and/or inert gases, is introduced into a fluidized bed reactor and thermally decomposed within the reactor to produce silicon which deposits upon seed particles inside the reactor.
A common problem in fluidized bed reactors is contamination of silicon-coated particles in the fluid bed at high operating temperatures by materials used to construct the reactor and its components. For example, nickel has been shown to diffuse into a silicon layer (e.g., on a silicon-coated particle) from the base metal in some nickel alloys used to construct reactor parts. Similar problems arise in fluidized bed reactors configured for pyrolytic decomposition of a germanium-bearing gas to produce germanium-coated particles.